1. Field of the Invention
The present invention relates to a system for controlling the gear changes of a vehicular automatic transmission and a method for executing the gear changes. More specifically, the present invention relates not only to a shift control system for an automatic transmission including series-connected first and second transmission assemblies, so that a downshift of the transmission may be achieved as a whole by simultaneously accomplishing a downshift for augmenting the gear ratio at the first transmission assembly and an upshift for reducing the gear ratio at the second transmission assembly, but also to a method for executing such shifts.
2. Discussion of the Background
In accordance with the rapid spread of the vehicular automatic transmission in recent years, there has been widely adopted, with a view to improving fuel consumption, a so-called "overdrive mechanism" having a gear ratio less than "1" which is connected as a second transmission assembly in series to a first transmission assembly capable of switching gear ratios automatically in relation to the running velocity and throttle opening of the vehicle.
In Japanese Patent Laid-Open No. 57-37140, there is disclosed an automatic transmission which is constructed to achieve six forward gear ratios by synchronizing the second transmission assembly or the overdrive mechanism positively with the shift of the first transmission assembly which is capable of achieving three forward gear ratios to shift the first and second transmission assemblies simultaneously or alternately. This automatic transmission is constructed to realize multiple gear ratios easily by setting the first transmission assembly at one of first to third speeds for each of the high and low-gear ratios of the second transmission assembly. In addition, the automatic transmission can achieve a number of advantages such as improvements in the fuel consumption and running performance and reductions in the loads upon frictional engagement means.
In this automatic transmission, however, the gear changes are performed by augmenting the gear ratio at the first transmission assembly and reducing the gear ratio at the second transmission assembly in the case of a downshift from the third to second speeds or from the fifth to fourth speeds. In other words, the gear changes are achieved by shifting the first and second transmission assemblies in the opposite directions to each other. If the gear changes, i.e., the shifts at the individual transmission assemblies were individually controlled in the case of such a downshift, the shifts could not be timed to augment shifting shocks. Another disadvantage would be an extraordinary drive feel which might otherwise be invited by a phenomenon that an upshift is caused at the second transmission assembly after a downshift at the first transmission assembly or vice versa.
On the other hand, we have revealed in Japanese Patent Laid-Open No. 62-165050 that excellent shifting characteristics can be obtained by starting and completing the substantial shift of the second transmission assembly during the substantial shift of the first transmission assembly. The substantial shift, as herein termed, is the period for which rotary members have their rotational frequency changed due to the shift, and is generally called the "inertia phase". Specifically, we have revealed in our Laid-Open specification that the start of the shift at the second transmission assembly should not occur before the start of the inertia phase of the first transmission assembly and that the end of the shift at the second transmission assembly should not occur after the end of the inertia phase of the first transmission assembly.
Let the case of a shift now be examined, in which the automatic transmission is to be shifted down in its entirety by performing a downshift for augmenting the gear ratio at the first transmission assembly and an upshift for reducing the gear ratio at the second transmission assembly. In this case, a timer is used to start the inertia phase of the second transmission assembly after the start of the inertia phase of the first transmission assembly. Generally speaking, the downshift has a smaller time lag from the output of a shift command to the actual start of the shift (i.e., the inertia phase) because it is achieved by releasing the frictional engagement means, whereas the upshift has a larger time lag from the output of the shift command to the start of the inertia phase because it is achieved by engaging the frictional engagement means. Thus, the timer is used for reflecting those time differences upon the shift controls. In case such a timer is used, the shift control system is constructed to output the upshift command having the larger time lag for the second transmission assembly and then outputs the downshift command for the first transmission assembly after a predetermined constant time has been counted by the timer. As a result, the inertia phase of the upshift of the second transmission assembly is started immediately after the inertia phase of the downshift of the first transmission assembly has been actually started.
In respect to the synchronous controls using such a timer, there has been disclosed in Japanese Patent Laid-Open No. 62-83541 a technology which enables more accurate synchronous controls to be accomplished by correcting the timings of the shift commands for the first and second transmission assemblies on the basis of the shift results performed beforehand.
In the systems disclosed in the above-specified Japanese Patent Laid-Open Nos. 62-165050 and 62-83541, the frictional engagement means of the second transmission assembly has a large time lag for its engagement so that the shift command is outputted too early to the second transmission assembly which should have its substantial shift started later. As a result, the output timing of the shift command to the first transmission assembly must be delayed due to the elongated time lag at the second transmission assembly. This raises a problem in that the time period for the shift to be completely ended from the decision of the shift of the automatic transmission is elongated because of the larger time lag at the second transmission assembly.
In order to solve these problems, it may be conceived to shorten the time lag by raising the oil pressure to be supplied to the frictional engagement means. With such a concept, however, the mere rise of the engagement oil pressure of the frictional engagement means of the second transmission assembly would cause another problem in that the shift shocks are augmented by the excessively fast engagement of the frictional engagement means in case the shift is to be achieved solely by the second transmission assembly.